Power amplifiers can be broadly classified into two categories, linear and non-linear, based upon the operation mode of the active elements they contain. Conventionally, the active elements are transistors although other active devices such as vacuum tubes have been used. In a linear amplifier, the active devices are maintained in a linear region of operation. Conversely, in a non-linear amplifier, the active devices are used as switches such that these amplifiers may also be denoted as “switch mode” or switching power amplifiers. Because a switch is ideally either fully on, with very low resistance (zero voltage across the switch) or fully off (zero current across the switch), the efficiency of switching power amplifiers is very good. In contrast, an active device in the linear region of operation is neither fully on nor fully off and it thus is always dissipating power through resistance to the resulting continual current flow. Switching power amplifiers are thus popular alternatives to linear amplifiers.
Although switching power amplifiers offer attractive power efficiencies, the behavior of their switches varies from the ideal zero voltage (when on) vs. zero voltage (when off) switch model. A real world switch requires some time to fully turn on/turn off and also has some appreciable resistance when fully on. Thus, switching power amplifiers are often configured to force the voltage across the switch to be effectively zero during the switching instances. Such modifications may be better understood with reference to FIG. 1, which illustrates a conventional switching power amplifier (SPA) 100. A transistor M1 is driven by a gate voltage Vin to switch on and off responsive to an input voltage Vin to be amplified by SPA 100. In this embodiment, M1 is an NMOS transistor although it will be appreciated that PMOS versions of SPA 100 may also be constructed as known in the amplifier arts. The drain of M1, denoted as node VA, couples to a power supply voltage node VCC through an inductor L1 that acts as an RF choke. Node VA also couples to an output node through a matching network 101 for supplying an output voltage Vout that represents the amplified version of input voltage Vin. Matching network 101 includes a capacitor C1 that couples between node VA and ground (VSS). A series connected RLC circuit 105 also couples between node VA and ground to complete matching network 101. RLC circuit 105 includes an inductor L2, a capacitor C2, and a resistor R1. The output voltage node is between capacitor C2 and resistor R1. Resistor R1 would thus be in parallel with a load for SPA 100 such that a conventional resistance for R1 would be 50 ohms.
To suppress non-idealities in the switching behavior of M1, matching network 101 functions to: (1) as M1 turns off, keep the voltage at node VA low long enough such that the current through M1 may drop to zero; and (2) as M1 turns on, keep the voltage at the node VA and its first derivative dVA/dt substantially at zero. The grounded capacitor C1 guarantees the first condition. Without capacitor C1, as the M1 turns off, the drain voltage VA would increase, introducing substantial power loss in transistor M1. To satisfy the second condition, the matching network consisting of C1, C2, L2, and R1 should operate as a damped second order system, with initial conditions across C1, C2, and L2. The first initial condition determines the value of C1 and the second initial condition determines the value of C2. But note that is conventional to drive M1 with a square wave input voltage Vin such that a resonant tank property of matching network 101 produces a corresponding sinusoidal output signal voltage Vout. A resonant circuit is resonant only at certain frequencies and is also characterized by a quality factor Q. Achieving a high resonant frequency and a high Q requires a relatively small capacitance in the resonant circuit. A high Q functions to reduce the harmonic distortion introduced in a sinusoidal output voltage Vout for SPA 100. Achieving low harmonic distortion in an SPA (such that its matching network has a relatively small capacitance) is thus at odds with minimizing switch non-idealities (which requires a relatively larger capacitance in the matching network).
Accordingly, there is a need in the art for a switching power amplifier that both minimizes harmonic distortion and suppresses switching non-idealities.